Effective method to improve sub-micron color filter sensitivity

Abstract

An image sensor device and method for forming said device are described. The image sensor structure comprises a substrate with photodiodes, an interconnect structure formed on the substrate, a color filter layer above the interconnect structure, a first microlens array, an overcoat layer, and a second microlens array. A key feature is that a second microlens has a larger radius of curvature than a first microlens. Additionally, each first microlens and second microlens is a flat convex lens. Thus, a thicker second microlens with a short focal length is aligned above a thinner first microlens having a long focal length. A light column that includes a first microlens, a second microlens and a color filter region is formed above each photodiode. A second embodiment involves replacing a second microlens in each light column with a plurality of smaller second microlenses that focus light onto a first microlens.

Description

FIELD OF THE INVENTION [0001] The invention relates to a solid state imaging device (hereafter referred to as a CMOS image sensor) and a method of manufacturing the same. More particularly, the image sensor has an improved microlens component that increases the sensitivity of the device. BACKGROUND OF THE INVENTION [0002] A complementary metal oxide semiconductor (CMOS) image sensor is a key component of many digital video cameras. The CMOS image sensor is typically comprised of an upper stack that includes one or more layers of color filters, a microlens array, and an overcoat for the microlens and a lower stack that includes interlevel dielectric (ILD) layers, interlevel metal (ILM) layers, and passivation layers which are fabricated on a substrate. High sensitivity is an important characteristic for a CMOS image sensor since the image quality may suffer if sensitivity is not high enough. The function of the microlens component is to focus light through a color filter layer and th...

AI Technical Summary

Benefits of technology

[0008] An objective of the present invention is to provide a microlens component in a CMOS image sensor that offers improved sensitivity in a device that has several metal (ILM) layers and requires a long focal length.

[0009] A further objective of the present invention is to provide a microlens component in a CMOS image sensor that offers improved sensitivity in a sub-micron device that has a pixel size smaller than about 5 microns.

[0010] A still further objective of the present invention is to provide a method of fabricating a CMOS image sensor having improved efficiency that is compatible with existing tools and materials and can be performed in a low cost manner.

[0014] A planar overcoat layer which is preferably comprised of the same transparent material as in the insulation layer is formed on the first microlens array. A post-exposure bake at 220° C. not only hardens the overcoat layer but also hardens the first microlens array. A second microlens array comprised of a plurality of second microlenses is fabricated on the overcoat layer by first patterning a positive tone photoresist, heating to reflow, and hardening with a 220° C. bake to form flat, convex shapes. A second microlens has a width that is essentially equal to the width of a first microlens. Moreover, there is a second microlens aligned over each first microlens. A second microlens preferably has a radius of curvature (H2) that is greater than H1. The larger radius of curvature provides for a shorter focal length and higher photon collection efficiency for a second microlens compared to a first microlens. Light that is efficiently collected by a second microlens is focused on a first microlens. The first microlens has a long focal length that is capable of focusing light through a light column within a thick lower stack of layers onto a photodiode.

[0015] The invention is also a microlens component of a CMOS image sensor with improved sensitivity that is formed on a lower stack of layers that includes a photodiode array on a substrate and a color filter layer as the top layer in the lower stack. The lower stack typically includes a interconnect structure on the substrate and an insulation layer formed on the insulation layer. The microlens component is comprised of a first microlens array formed above the color filter layer, a planar transparent overcoat layer on the first microlens array, and a second microlens array formed on the overcoat layer. Each of the plurality of microlenses in the first microlens array and in the second microlens array has a flat convex shape. An important feature is that the radius of curvature of a second microlens is greater that the radius of curvature of a first microlens. A light column is formed which involves a vertical arrangement of a photodiode, a color filter region, a first microlens, and a second microlens. Other intermediate layers described previously have a high transmittance to minimize loss of light intensity as light passes through the light column. Photon collection efficiency (ψ) of a microlens is expressed by the equation ψ=4PI×R2 where PI is the photorefractive index of a microlens and R is the radius of curvature.

Problems solved by technology

High sensitivity is an important characteristic for a CMOS image sensor since the image quality may suffer if sensitivity is not high enough.

The longer focal length is a big challenge to maintaining adequate sensor sensitivity.

Since the lower microlens array does not reside on a planar surface, the size and oval shape of the microlens can be difficult to reproduce.

In addition, the poly(vinylphenol) / diazonaphthoquinone diazide (DNQ) based photoresist that comprises the lower microlens has less suitable dissolution characteristics for patterning than a more common Novolak resin / DNQ photoresist.

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